Physicists in Japan have discovered Carbon-22, an exotic isotope of one of the universe's most abundant elements. With a nucleus greater than those of heavier elements like carbon and zinc, it is the biggest example yet of a "halo nucleus."
Discovered in a particle accelerator at the University of Tokyo, Carbon-22 has six protons and a whopping sixteen neutrons in its nucleus. In most such overstuffed isotopes, the excess neutrons create instability and cause the atom to break apart. However, Carbon-22 makes use of a unique arrangement of its nucleus that gives it stability unusual for its size.
While the next biggest carbon isotope, Carbon-21, has a half-life of under 30 nanoseconds - or billionths of a second - Carbon-22 has the relatively long-lived half-life of roughly 6 milliseconds. That might not sound like a lot of time, but it still means Carbon-22 holds together for about a million times the average half-life of Carbon-21.
The reason for this is that Carbon-22 arranges itself in what is called a halo nucleus. In this arrangement, two of the neutrons detach from the nucleus and orbit around it, forming a halo around the nucleus. This essentially alters the nucleus so that it is now a three body system, with a core of six protons and fourteen neutrons along with the two orbiting neutrons. Although physicists still don't exactly know why, this arrangement provides the needed stability for Carbon-22 to hold together longer.
This phenomenon is mysterious because physicists still can't easily model halo nuclei; the mathematics involved are just too hellishly complicated. In the meantime, scientists have taken to nicknaming these arrangements "Borromean nuclei" after the fifteenth century rings. The Borromean rings were three rings interlocked such that cutting any one ring will cause all three to separate. This is in reference to the fact that, for whatever reason, the halo nucleus configuration is the only arrangement that gives Carbon-22 stability, and even the slightest deviation will make it fall apart instantly.
Physicists hope the discovery of Carbon-22 is just the beginning of a far larger exploration of exotic isotopes, as new and better detectors will allow scientists to discover even larger isotopes in halo nucleus configurations.